Although transversal equalizers have been used for many years in communication networks, only recently have their applicability to ultra-high bit rate optical communication systems been realized as a means to improve receiver sensitivity. See, for example, B. Kasper et al., J. Lightwave Technology, Vol. LT-5, pp. 344-47 (1987). In particular, conventional transversal equalizers, which comprise several variable delay taps and amplifiers in parallel, have been utilized to decrease the intersymbol interference caused by the non-ideal transmission characteristics of the associated channel that results from non-uniform gain and delay over the frequency band of the channel.
In general, a conventional transversal equalizer divides an incoming electrical signal into several branches, effectuated by a series of resistive networks. Each branch then delays and amplifies or attenuates the signal therein by a variable amount so as to counteract and correct for distortion within the original signal. For example, a pulse transmitted from a non-ideal channel may have several overshoots and undershoots. An undershoot may be canceled by adding to the original pulse an inverted and delayed replica of itself through the use of resistive combiner networks. Depending on the pulse response of the linear channel, the polarity and delay within each branch may be selected accordingly to cancel any number of undershoots or overshoots. In other words, amplification and delay within each branch may be selected such that the overall impulse response of the channel and the transversal equalizer has substantially an ideal pulse response. For a more detailed discussion of signal filtering with transversal equalizers, see, for example, IEEE. Proc. 7th Allerton Conference On Circuit System Theory, pp. 792-9 ( 1969).
Generally, conventional transversal equalizers require wide band, high gain amplifiers within each branch to compensate for the attenuation resulting from the required lossy components, such as resistive splitter and combiner networks. Although these amplifiers serve to improve the isolation between the outputs and inputs of each branch, their prohibitively large size, cost and non-cascadability make integration problematic. Additionally, the structure is substantially complicated because each amplifier typically requires a high power current supply in order to provide sufficient gain for compensating signal attenuation therein. While prior art transversal equalizers have performed acceptably, the overall performance has been further limited and critically dependent on the non-ideal characteristics of these wide band, high gain amplifiers, such as gain and phase ripples.